Blood vessels form

a closed system composed of arteries, arterioles, capillaries, venules, and veins.

An artery is a

vessel taking blood away from the heart

 Vein

is a vessel returning blood to the heart.

Walls of arteries and veins have three coats

A. Tunica intima

Tunica intima

inside lining of vessel

Tunica intima - 2 layers

1. Layer of simple squamous endothelium

2. Layer of elastic fibers (found only in artery)

Tunica media

layer of smooth muscle and elastic fibers

(veins have more connective tissue fibers mixed with the smooth muscle)

Tunica externa or tunica adventicia

connective tissue located on the outside.

Arteries - Characteristics

1. Walls are thicker and contain more smooth muscle.

2. Lumen is round (particularly in smaller vessels)

3. Valves are absent

Types of arteries

1. Elastic or conducting arteries

2. Distributing or muscular arteries

Elastic or conducting arteries

large vessels that contain more elastin and muscle fibers. (Aorta and its direct branches)

Distributing or muscular arteries

distributes blood to various parts of the body and is composed of a thick tunica that constricts and dilates

Arterioles

are small arteries that contain all three coats on the arterial end, but are composed of an endothelial lining with a few smooth muscle cells towards the capillary end

Capillaries - size

Smallest are 7-10 microns in diameter

are composed of a single layer of simple squamous epithelium with a basement membrane

Amount of blood flowing through a capillary is determined by 

a precapillary sphincter muscle located at the origin of the capillary

With few exceptions a capillary should be located within

60-80 microns of any tissue cell

Capillaries contain approx. 

250 ml. of blood out of the 5000 ml. of blood flowing through the vessels at any time

Capillaries function

as the area of exchange between the blood and the body tissues

Types of Capillaries

1. Continuous

Continuous

adjacent endothelial cells closely joined. 

Fenestrated

capillaries contain large pores covered with the basement membrane.

Located in the kidneys, endocrine glands, intestines, and choroid plexus of brain

1. have less smooth muscle and more connective tissue

2.  lumen partially collapsed

3. contain valves

1.  return blood to heart

2.  act as blood reservoirs (approx. 2/3 of blood volume located in veins)

End Arteries

blood pressure

1. cardiac output

2. blood volume

3. peripheral resistance

Blood volume

directly proportional to blood pressure. Anything that increases blood volume increases blood pressure.

1. ADH

2. Aldosterone

Peripheral resistance

resistance to blood flow by force of friction between blood and walls of vessels

Any condition that increases viscosity increases 

blood pressure

The smaller the diameter of the vessel

 the more resistance

The longer the vessel

the more resistance

How many additional miles of capillaries are made for each pound?

200 additional miles of capillaries

Control of Blood Pressure

Increase in heart rate and muscular contractions increases blood pressure.

Factors controlling heart rate and force on contractions also affect?

affect blood pressure

Factors acting on blood vessels

1. Vasomotor center in medulla controls diameter of blood vessel.

2. Pressoreceptors in aortic arch and carotid sinus

3. Chemoreceptors in aortic arch and carotid sinus (monitors low oxygen by checking for high hydrogen ions. High carbon dioxide levels lead to high hydrogen ion concentrations. This causes vasoconstriction of non-essential vessels.

4. Higher brain centers - emotions

5. Chemicals

6. Autoregulation - Oxygen is the principal stimulant. Low oxygen causes vasodilator substances to be released.

Examples are K^{+ ,} H^{+ ,} CO_{2 ^{, lactic acid, and adenosine. This causes localdilation and relaxation of precapillary sphincter muscles.}}

Highest in venules (16 mm Hg) and lowest at junction of vena cava with right atrium (0 mm Hg)

Aids venous blood returning to heart

2. Sympathetic nerve activity

3. Skeletal muscle pump

4. Pressure difference between thoracic and abdominal cavities

Velocity of blood in inversely related to TOTAL cross-sectional area

Aorta

Capillaries

Vena cava

Aorta

cross-sectional area of 2.5 cm² and velocity of 40 cm/sec.

Capillaries

cross-sectional area of 4500 - 6000 cm2 and velocity of .1 cm/sec.

Vena cava

cross sectional area of 8 cm₂ and velocity of 5 to 20 cm/sec.

Distribution of Blood

1. systemic veins

2. systemic arteries

3. Capillaries

4. Head and lungs

60%

1.  Act as blood reservoirs

2. veins have thinner, less muscular walls, therefore higher compliance.

Pulmonary

all deoxygenated blood leaving the right ventricle and returning to left atrium